May in the lower elevations (like Denver) will be mostly ok, temps in the 60-70's. At alitudes, above 7000', it could be cool, even during the day. May is also a very fickle month, we had a serious snowstorm a couple years back about mid-month. Afternoon thunderstorms are also close to their annual peak and are usually brewing by about 3 pm. Trail Ridge Road, which passes through Rocky Mountain National Park, is usually open by mid-May but the snow banks will dwarf the FJR. The southern half of the state is definitely warmer, however around, or in, the San Juan Mountains (Telluride, Ouray,Ridgeway) snow will still be present and the afternoon boomers will be common. Along the western border with Utah the weather will be much more consistent,and offers a refuge should the weather turn cold or white. By all means, expect cool, NOT hot temps. Snow above 9000' (which is alot of the mountains!!) is not uncommon til June. In fact, very few mountain passes are below that elevation.